HEARING. 301 



solution of common salt (saline), and a 1 per cent, solution of 

 acetic acid (acid). 



(a.) Wipe the tongue dry, lay on its tip a crystal of sugar. 

 It will not be tasted until it is dissolved. 



(b.) Apply a crystal of sugar to the tip and another to the 

 back of the tongue. The sweet taste is more pronounced at 

 the tip. Do the same with the sugar solution, applying it 

 by means of a small camel's-hair brush. 



(c.) Repeat the same with sulphate of quinine in powder 

 and in liquid. It will scarcely be tasted on the tip of the 

 tongue, but will be tasted immediately on the back part 

 of the dorsum. 



(d.) Ascertain where saline and acid substances are tasted 

 most acutely. 



(e.) Connect two zinc terminals with a large Grove's 

 battery, apply them to the upper and under surface of the 

 tongue, and pass a constant current through the tongue. 

 An acid taste will be felt at the positive, and an alkaline 

 one at the negative pole. 



(f.) Close the nostrils, shut the eyes, and attempt to 

 distinguish by taste alone between an apple and a potato. 



8. Hearing. 



(a.) Hold a ticking watch between your teeth, or touch 

 the upper incisors with a vibrating tuning-fork, close both 

 ears, and observe that the ticking is heard louder. Unstop 

 one ear, and observe that the ticking is heard loudest in the 

 stopped ear. 



(b.) Hold a vibrating tuning-fork on the incisor teeth 

 until you cannot hear it sounding. Close one or both ears 

 and you will hear it. 



(c.) Listen to a ticking watch or a tuning-fork kept 

 vibrating electrically. Close the mouth and nostrils, and 

 take either a deep inspiration or deep expiration so as to 

 alter the tension of the air in the tympanum; in both cases 

 the sound is diminished. 



